The present invention relates to a current switch for monitoring electric current in a conductor and, more particularly, to a self-calibrating current switch having a display.
Industrial and commercial environments utilize large numbers of electrically powered devices, such as heaters, valves, fans, pumps and compressors. The proper operation of these devices can be important to the protection of valuable property and successful completion of processes that may involve costly or hazardous equipment, operations or materials. The operation of these devices is commonly monitored by current switches which include a current transducer which outputs a signal that is representative of the current flowing in a conductor supplying power to the monitored device(s) and a sensing switch that changes state when the signal from the current transducer indicates that the current in the monitored conductor has crossed a trip point, sometimes referred to as a set point, for the switch. The current switch is typically connected to a controller which, when the state of the sensing switch changes, may display a warning or an advisory signal for a human operator and/or selectively enable or disable power to the device and/or other devices of a system that might be affected by a malfunction of a device connected to the conductor monitored by the current switch.
Holce et al., U.S. Pat. No. 5,808,846, incorporated herein by reference, disclose a protective device comprising a current sensor for monitoring current in a cable supplying power to a load and an input circuit for signaling a controller when the current reaches a trip point. The current sensor is a sensing transformer comprising a wire wound core that encircles a power cable. A changing current in the power cable, the primary winding of the sensing transformer, produces a changing electro-magnetic field around the cable which, in turn, induces a magnetic flux in the core of the transformer. The magnetic flux in the core induces a voltage in the wire winding, the transformer's secondary winding, which is representative of the current flowing in the power cable. The wire winding is electrically connected to the input circuit that outputs a signal to a controller when the voltage signal in the secondary winding of the transformer indicates an improper current in the power cable. The protective device also includes a status output which can energize or deenergize the monitored device or another device in response to a signal from the controller.
Current switches are available with either a fixed trip point or an adjustable trip point. A current switch with a fixed trip point provides cost effective “go/no go” monitoring but is usually unsuitable for monitoring circuits for events which can cause the electrical load, for example, a motor, to continue to operate but at either reduced or increased current draw. For example, belt loss or shearing of a coupling connecting a motor and a mechanical load can cause the mechanical load to malfunction while the electrical load, the motor, continues to operate with a reduced current. On the other hand, a mechanical failure, such as a bearing failure, will often cause an increase in the current draw of a motor for some period before the motor stops operating. The trip point of some current switches can be adjusted to enable detection of low or high currents indicative of these types of failures. Typically, the system is started and allowed to achieve steady state operation. Then the installer adjusts a potentiometer in the current switch to vary the trip point until the state of the sensing switch changes indicating that the trip point is equal to the normal operating current in the monitored conductor. Continued adjustment of the potentiometer adjusts the trip point to a current that is either somewhat greater than or somewhat less than the normal operating current. Thereafter, the sensing switch will change state when the current in the monitored conductor crosses the trip point. While current switches with adjustable trip points provide greater utility, adjusting the trip points of the current switches is time consuming and installation can be costly, particularly when large numbers of current switches are involved. In addition, while the trip point is known to be a value which is either greater or less than the conductor current when the system is operating normally, additional instrumentation and monitoring is required to determine the magnitudes of the conductor current and the trip point(s) of the current switch if the trip point is a function of the conductor current. Moreover, the magnitude of a current causing the current switch to trip is not readily knowable which can complicate troubleshooting particularly if the current switch has both an upper and a lower trip point.
What is desired, therefore, is a current switch that self-calibrates to a current in the monitored conductor and enables a user to visually determine the current in the conductor and the trip point of the current switch.